1. Field of the Invention
The present invention relates to a power tool, and specially to a power tool, such as a circular saw, a grinder, a router, a sander, a miter saw and a cutting machine etc, which rotates in a single direction and can lock the output shaft when changes the work element.
2. Description of the Prior Art
A power tool, such as a circular saw, a grinder, a sander, a miter saw and a cutting machine etc, usually drives output shaft rotating by a motor, and output shaft mechanism drives a work element rotating to cut or polish. However, after a long working, the work element is easy to be worn out and need to be changed. Furthermore, the operator will change various work elements to fit various workpieces. But when use a wrench or such tool to disassemble the work element, the output shaft will rotate with the wrench accompanying, thereby it is inconvenient to disassemble the work element. The prior art has disclosed various locking device to prevent the output shaft rotating with the wrench. U.S. Pat. No. 5,430,944 discloses a circular saw which comprises a disc with a slot fixedly connected to a output shaft, and a locking pin located in a housing which can be pressed into the slot to prevent the output shaft rotating by an actuating pole manually sliding-operated by an operator. When the operator releases the actuating pole, the locking pin is biased to withdraw from the slot by a torsion spring and permit the output shaft rotating. But the output shaft locking mechanism has more components, and more complex configuration, and such a manual operation requires a hand of the operator to operate the actuating pole for locking the output shaft and the other hand to disassemble the work element with a wrench. Such a manual operation is arduous and not friendly to the operator. DE Patent No. 19938299A1 discloses a circular saw which comprises a spindle locking mechanism located in an axial hole of a drive spindle such that a wrench is inserted into the hole to disassemble a work element, which constantly pushes the output shaft locking mechanism axially moving to mesh with a motor fan for locking the output shaft. When taking out the wrench, the output shaft locking mechanism is biased to release the drive spindle by a torsion spring. Such output shaft locking mechanism, which uses a wrench to disassemble the work element and lock the output shaft simultaneously, is very dangerous specially as an operator forgets to take out the wrench and actuates the motor, the wrench maybe thrown out in a high speed to hurt the operator, or the wrench still prevents the output shaft rotating so as to hurt the motor.